Conventionally, variable intake systems include technology for switching the length or cross-sectional area of an intake pipe of a multiple cylinder engine between two stages according to factors such as the engine speed and the engine load. The systems apply an inertia supercharging effect or resonance supercharging effect to the intake air to improve the output of the multiple cylinder engine. Examples of such systems are disclosed in Japanese Patent Laid-Open Publication No. Hei 07-166877 at pages 1 to 5, FIG. 1 through FIG. 7; Japanese Patent Laid-Open Publication No. Hei 08-277717 at pages 1 to 9, FIG. 1 through FIG. 9; and Japanese Patent Application No. 2002-317718 at pages 1 to 4, FIG. 1 through FIG. 7. In such a variable intake system, as shown in FIG. 9 and FIG. 11, a butterfly valve bearing support structure that supports multiple integrated butterfly valves 104 in a freely rotatable manner (a conventional embodiment) is provided in a valve bearing section 103 of an intake manifold 102, and switches the length or cross-sectional area of intake air passages 101 connecting to each cylinder in a multiple cylinder engine.
Here, the butterfly valves 104 are supported by a single valve shaft 105 with a rotation center axis oriented in a direction orthogonal to the center axis direction of the intake air passages 101. Furthermore, opening and closing control of the butterfly valves 104 is performed by an actuator 110 to rotationally drive the valve shaft 105. A valve axis (effectively, a bearing section) 106 which covers the outer periphery of the valve shaft 105 is formed in an integrated manner at each end, in the rotation center axis direction, of each butterfly valve 104. Furthermore, the end of the valve shaft 105 nearest the actuator 110 is supported by a bearing 107 or the like, for example.
In recent years, from the viewpoints of reducing weight and improving the insulation efficiency and the level of design freedom, various trials and proposals have arisen regarding the use of a resin to form the intake manifold 102 and the butterfly valves 104, for example. However, when the components of the intake system of an engine are made of a resin, the level of molding precision is lower than that observed for metallic materials. Consequently, if there is little bearing clearance between the inner circumferential surface of the valve bearing section 103 of the resin intake manifold 102 and the outer circumferential surface of the valve axis 106 of the resin butterfly valve 104, then the sliding resistance between the valve bearing section 103 and the valve axis 106 increases. To suppress this increase in sliding resistance, it is necessary to ensure a large bearing clearance between the valve bearing section 103 and the valve axis 106. As a result, the end of the valve shaft 105 and the valve axis 106 on the opposite side to the actuator (the left side in the diagram) is a free edge, the movement of which, in a radial direction orthogonal to the rotation center axis direction, is not constrained by the valve bearing section 103.
However, intake air, which flows through the plurality of intake air passages 101, includes pulsation introduced by the rotation of the engine. Consequently, this pulsation causes each of the butterfly valves 104 provided in one of the plurality of intake air passages 101 to vibrate. In particular, when the intake pressure which accompanies the pulsation acts on the butterfly valve 104 on the opposite side to the actuator, namely, the side with the free edge, the end of the valve shaft 105 on the opposite side to the actuator undergoes a large amount of vibration. As a result, the collision speed increases between the valve bearing section 103 of the resin intake manifold 102 and the valve axis 106 of the resin butterfly valve 104. Furthermore, the outer circumference of the valve shaft 105 is covered by the valve axis 106 of the resin butterfly valve 104. Consequently, at the end of the valve shaft 105 on the opposite side to the actuator, contact occurs between the valve bearing section 103 of the resin intake manifold 102 and the valve axis 106 of the resin butterfly valve 104.
In regions where components made of a resin material contact each other, the critical PV value, which indicates slipperiness, is low. Consequently, the slidability between the valve bearing section 103 of the resin intake manifold 102 and the valve axis 106 of the resin butterfly valve 104 worsens, resulting in an extremely large amount of abrasion (see FIG. 5). As the abrasion between the valve bearing section 103 of the resin intake manifold 102 and the valve axis 106 of the resin butterfly valve 104 progresses, the bearing clearance between the valve bearing section 103 of the resin intake manifold 102 and the valve axis 106 of the resin butterfly valve 104 increases. As a result, rocking of the valve shaft 105 accompanying vibration increases, and the collision speed of the valve bearing section 103 with the valve axis 106 increases further, leading to a problem of abnormal noise generation.
Therefore, with an object of improving abrasion resistance between the valve bearing section 103 of the resin intake manifold 102 and the valve axis 106 of the resin butterfly valve 104, the inventors of the present invention have already filed Japanese Patent Application No. 2002-219765 on Jul. 29, 2002. Japanese Patent Application No. 2002-219765 discloses the bearing hole 111 side of the valve bearing section 103 of the resin intake manifold 102 being replaced with a metal sleeve 112, as shown in FIG. 10 and FIG. 12. However, because the bearing structure comprises both parts made of a metal and parts made of a resin, the level of tapping noise generated at the bearing clearance between the inner circumferential surface of the metal sleeve 112 and the outer circumferential surface of the valve axis 106 of the resin butterfly valve 104 is higher than in a bearing structure comprising only parts made of a resin, and it is therefore necessary to reduce the bearing clearance.
As a result, reducing the bearing clearance between the inner circumferential surface of the metal sleeve 112 and the outer circumferential surface of the valve axis 106 of the resin butterfly valve 104 causes the slidability and abrasion resistance between the metal sleeve 112 and the valve axis 106 of the butterfly valve 104 to deteriorate. As a countermeasure, the metal sleeve 112 is bonded to the valve bearing section 103 via an elastic body (such as fused rubber) 113. However, in this type of a butterfly valve bearing support structure (a related embodiment), the number of components and the number of assembly steps increases, which may result in lower productivity and higher costs.